igbv: Do not replay VLANs while stopped

iflib clears IFF_DRV_RUNNING before the driver stop callback but leaves
IFF_DRV_OACTIVE set. Consequently, an already queued admin task can
run after the VF reset. If that task consumes a pending timer sample,
it can retry failed VLAN mailbox operations and restore PF filters for
the stopped VF.

Continue sampling statistics, but only run the VLAN retry worker while
the interface is running.